At the request of Labohm, it was decided to use the UAH satellite temperature data set on the lower troposphere (TLT) (roughly the lowest 5 km of the atmosphere). These data sets are compiled by the University of Alabama in Huntsville. Satellites are used to measure radiation in the atmosphere, after which the temperature of the various layers of the atmosphere is derived using a complex algorithm.

According to the UAH today, temperatures appear to have been an average 0.1 °C warmer over the past five years than over the 10 years before that. Thus, Strengers has won the wager. The stakes: a good bottle of wine.

The UAH temperature series since 1979 (no satellites were available for the period before then). The green lines represent the mean over periods of 10 years. The purple line on the far right is the mean over the 2010–2014 period.

UAH satellite data series shows the greatest warming

Precisely these UAH data, incidentally, show by far the most warming. The 4 other main global temperature series also show warming over the last 5 years, but one that is markedly lower (between 0.03 and 0.05 °C).

What causes the differences between the data series?

The table below shows the global warming, in °C, over the past 5 years, compared to the 10 years before that, for the five main global temperature series: the satellite series of the University of Alabama in Huntsville (UAH) and of the Remote Sensing Systems (RSS), and the surface temperature series of NASA, Climate Research Unit (CRU) and the National Climatic Data Centre (NCDC). CRU’s series are based on surface temperature measurements up to and including November 2014, as data on December were not yet available.

The large difference (by more than a factor of 3!) between the UAH and RSS satellite series is remarkable (also see the graph below). According to the UAH team, in which two well-known climate sceptics are involved, the difference is mainly caused by the fact that RSS partly bases its series on an old satellite (NOAA-15) with an increasingly lower orbit around the earth. This causes an error in measurements that is insufficiently corrected by RSS. All in all, it is a technical and complex issue, which possibly causes the differences, but it mainly shows how complicated the procedure is for determining global temperatures on the basis of satellite measurements. The three surface measurement series provide a much more consistent image of between 0.04 °C and 0.05 °C warming.

In addition, it is important to note that satellite and surface measurements are difficult to compare. This is due to the fact that satellite series are based on the temperature of the entire lower troposphere (the lowest 5 km of the atmosphere). The temperature of this atmospheric layer is, for example, much more sensitive to El Niños than surface temperatures are. This is illustrated in the graph below by the relatively high peak for the two satellite series at the time of the super El Niño in 1997–1998 and the less strong El Niño of 2010. The reverse is the case for La Niñas, such as the strong one of 2008; here, satellite series typically show a lower temperature.

Temperatures according to 2 satellite series (UAH and RSS). The purple line indicates the mean of the three surface temperature series. The satellite series show peaks in 1998 and 2010, as a result of El Niño, which are greater than those in the surface temperature series. The low satellite value for 2008 coincides with the opposite of an El Niño: La Niña. Note how the last 4 years in the RSS series are far below those in the other series. According to the surface temperature measurements, 2014 was the warmest year on record!

The graph shows that the last years in the RSS series clearly deviate from the other temperature series, with lower values of over 0.1 °C. This suggests that RSS rather than UAH is too low (as also claimed by the UAH team). The outcome of this discussion may lead to adjustments to one or both satellite series, as has been done in the past, particularly to the UAH series, on numerous occasions.

Strengers indicated at the time that ‘in light of the scientific uncertainties, I may lose, but this is not likely to happen’. He gave four reasons why a possible reduction in warming, or even a cooling could occur. Bold indicates that the related reason more or less became a reality over the past 5 years.

a continued (relatively) low solar activity;

a relatively high heat absorption by the (deep) oceans;

a period of cooling due to incidental variations in the climate;

lower climate sensitivity than expected.

In addition, Strengers gave three reasons why he nevertheless expected to win:

a further increase in greenhouse gas concentrations in the atmosphere;

the ‘best-estimate’ by the IPCC is that of a warming of 0.2 °C per decade;

the chances of overestimating climate sensitivity are smaller than those of underestimation.

The sum of all factors, thus, has led to continued warming. Below each of these factors is explained in more detail.

Continued (relatively) low solar activity

Over the past 5 years, the reduced solar activity has continued and, thus, likely also has slightly reduced global warming over that period. In the discussion at the time, Strengers wrote: ‘astrophysics […] cannot rule out the possibility of a long period of relatively low activity. This could lead to a reduction in warming of up to 0.4 °C (although 0.2 °C is more likely) over the coming 20 to 30 years.’ The past 5 years, therefore, are in keeping with the idea that such a period of relatively low activity is a fact, but the degree to which this reduction will actually continue over the coming years, or for how long it will go on, is still very uncertain.

Relatively high heat absorption by the (deep) oceans

Over 90% of the heat that is added to the climate system, particularly caused by the increase in greenhouse gases, ends up in the oceans. Only a few per cent is stored in the atmosphere. The remainder is absorbed by the land surface and ice sheets (which are therefore steadily melting). Variations in heat absorption can have a large impact on surface temperatures. According to a recent study by England et al., published in December 2013 in Nature, there has been increased heat absorption by the oceans since 2001, which since then has reduced warming by 0.1 to 0.2 °C. The added heat seem to be concentrated largely around the equator in the western part of the Pacific Ocean, at a depth of around 125 to 200 metres, which means it remains ‘hidden’ from the atmosphere. England and his team do not expect this heat storage effect to continue in this way and they project that, at a certain moment, temperatures at the surface level will begin to increase more rapidly. This could happen, for example, due to an El Niño with large amounts of heat being released suddenly, possibly causing temperatures to jump, as happened in 1997–1998 during the so-called super El Niño. Over the past months, a new El Niño seems to be developing. If this continues into 2015, this year may end up being even warmer than the record year of 2014.

A period of cooling due to incidental variations in the climate

The climate knows random variations. Strengers wrote that these may lead to longer periods of no warming or even cooling, even under a steady increase in greenhouse gas concentrations in the atmosphere. During the discussions, Strengers pointed to a study which shows on the basis of climate models that periods of up to 16 years of random cooling or non-warming may occur, even in an overall warming climate. Recent research shows that a combination of random factors likely has led to a reduction in temperature increases over the past 15 years (see the section below, ‘IPCC’s ‘best-estimate’ is that of a warming of 0.2 °C per decade’, for more details). However, this reduction in warming was not high enough for the past 5 years to be cooler than the decade before that.

Lower climate sensitivity than expected

The IPCC – the scientific body that inventories all knowledge on climate change every 5 to 7 years –stated in 2007 in its fourth assessment report (AR4) that climate sensitivity was likely (i.e. with a likelihood of 66%) between 2.0 and 4.5 °C, with a ‘best estimate’ of 3 °C. The fifth assessment report (2013) stated a range of 1.5 to 4.5 °C without giving a ‘best estimate’. The reason for the downward adjustment of the lower limit to 1.5 °C (at which it had been estimated since 1990) originated from a number of studies that pointed to the possibility of a low climate sensitivity. The ‘best estimate’ was not provided “because of a lack of agreement on values across assessed lines of evidence and studies” (i.e. based on all studies up to and including July 2012). All this, however, does not mean that climate sensitivity was ‘less than expected’. In fact, the only thing that can be concluded is that the value of climate sensitivity has become more uncertain.

Further increase in greenhouse gas concentrations in the atmosphere

Greenhouse gas concentrations in the atmosphere have steadily increased over the past 5 years. By late 2014, CO2 concentrations were at 399 ppm (399 molecules of CO2 per million molecules of air). Five years ago this level was 388 ppm. The increase is a direct result from an ever faster increase in CO2 emissions, particularly in countries such as India and China.

IPCC’s ‘best-estimate’ is that of a warming of around 0.2 °C per decade

At the time of IPCC’s fourth assessment report, in 2007, a global warming of 0.2 °C was assumed for the current decade (2010–2019), particularly on the basis of climate model results. As discussed above, the degree of warming according to the UAH series, which is based on satellite measurements, was 0.1 °C over the last 5 years, compared to the mean of the 10 years before that. If this trend continues over the coming 5 years, our current decade will register a warming of around 0.15 °C – slightly less than the ‘best estimate’, but well within the projected range by the IPCC. However, all surface temperature series show a lower degree of warming, between 0.04 and 0.05 °C, over the past 5 years (see the section on ‘What causes the differences between the data series?’). Extrapolation over the 2010–2019 decade shows a total maximum warming of 0.08 °C [typo fixed]. This is in line with the discussion on the ´hiatus´ or the finding that the rate of warming over the past 15 years has been lower than in the 20 years before that, and also lower than the average outcome of many climate models. Note though that there is no significant change in trend from 1998. If climate model calculations take into account the ´random factors´ that cannot be predicted, such as the occurrence of El Niños, solar activity, and volcano eruptions, then models and observations seem much more in agreement.

The chances of overestimating climate sensitivity are smaller than those of underestimation

The IPPC’s fifth assessment report (2013) states that climate sensitivity is likely (66% probability) to be between 1.5 and 4.5 °C. It subsequently states that it is extremely unlikely (less than 5% probability) to be smaller than 1, and very unlikely (less than 10% probability) to be higher than 6. In other words, very low values are less likely than very high values, which substantiates the above statement.

Collin wrote a blogpost about it as well which is well worth a read, giving a bit of context from other opinion and literature surveys.

The interview starts off with the general findings regarding the level of consensus, then focusing on how this compares with previous studies, how the media coverage is slanted towards contrarian views, and he gives me a chance to talk about my favorite part, how aerosol cooling masks the greenhouse warming and how this makes the phrasing of the IPCC AR4 attribution statement, by focusing only on the greenhouse warming part, prone to being misinterpreted. These aspects were also discussed in my blogpost from last month.

The PBL aimed to characterize the spectrum of scientific opinion about physical climate science issues. The research was focused on issues that are a frequent topic of public debate, and explored questions such as:

On which issues is there widespread agreement amongst scientists?

On which issues do scientists hold varied opinions?

How does the spectrum of scientific opinion compare to IPCC assessments?

How do scientists view skeptical arguments and viewpoints?

2. What is the relevance of an opinion survey or of measurement of consensus in trying to assess the science?

Science is based on the critical evaluation of available evidence in the context of existing knowledge. It is not “just an opinion.” With this survey, we tried to identify how scientists assess the different viewpoints that exist in public discussions of climate science. If the evidence for a certain viewpoint has become sufficiently strong and stable over time, the scientists’ aggregated opinion could be expected to reflect that.

3. Are the survey results publicly available?

The full survey results are not publicly available, because the PBL intends to use the data for further analyses.Update:
The ‘straight counts’ for every question (i.e. the number of responses for each answer option) will be made publicly available in the near future. This is not segregated in different sub-groups.

Conclusions

4. How does this study compare to the often-quoted 97% consensus?

Our results are consistent with similar studies, which all find high levels of consensus among scientists, especially among scientists who publish more often in the peer-reviewed climate literature.

Cook et al. (2013) found that 97% of papers that characterized the cause of recent warming indicated that it is due to human activities. (John Cook, the lead author of that analysis, is co-author on this current article.) Similarly, a randomized literature review found zero papers that called human-induced climate change into question (Oreskes, 2004).

Other studies surveyed scientists themselves. For instance, Doran and Kendall-Zimmermann (2009) found lower levels of consensus for a wider group of earth scientists (82% consensus) as compared to actively publishing climatologists (97% consensus) on the question of whether or not human activity is a “significant contributor” to climate change. Our results are also in line with those of e.g. Bray and von Storch (2008) and Lichter (2007).

In our study, among respondents with more than 10 peer-reviewed publications (half of total respondents), 90% agree that greenhouse gases are the largest – or tied for largest – contributor to recent warming. The level of agreement is ~85% for all respondents.

While these findings are consistent with other surveys, several factors could explain the slight differences we found:

Surveys like ours focus on opinions of individual scientists, whereas in a literature analyses the statements in individual abstracts are tallied. Literature analyses have generally found higher levels of consensus than opinion surveys, since the consensus is stronger amongst more heavily published scientists.

This study sets a more specific and arguably higher standard for what constitutes the consensus position than other studies. For instance, Doran and Kendall-Zimmermann (2009) asked about human activity being a “significant contributor” to global warming, and Anderegg et al. (2010) investigated signatories of public statements, while we asked specifically about the degree to which greenhouse gases are contributing to climate change in comparison with other potential factors.

Contrarian viewpoints are somewhat overrepresented in our survey and they may have overestimated their self-declared level of expertise (see question 9).

5. How is the consensus or agreement position defined?

The consensus position was defined in two ways:

Greenhouse gases contributed more than 50% to global warming since the mid-20th (Question 1). This is analogous to what was written in IPCC AR4.

Greenhouse gases have caused strong or moderate warming since pre-industrial times (Question 3). “Moderate” warming was only interpreted as the consensus position if no other factor was deemed to have caused “strong” warming. This response means that greenhouse gases were considered the strongest –or tied for strongest- contributor to global warming.

The former definition exactly mirrors the main attribution statement in IPCC AR4 and served as a ‘calibration’ for the latter.

6. What does “relative response” mean on the y-axis of many Figures?

This gives the percentage of the respondents (often within a certain sub-group) for the specific answer option. We opted to show the relative response rather than the absolute response to enable comparing the responses of different sub-groups (with differing group sizes as denoted by N=…) within one graph.

7. What are “undetermined” answers?

Those are the sum of responses “I don’t know”, “unknown” and “other”.

8. Why do IPCC AR4 authors show a higher consensus than the other respondents?

AR4 authors are generally domain experts, whereas the survey respondents at large comprise a very broad group of scholars, including for example scientists studying climate impacts or mitigation. Hence we consider this to be an extension of the observation -in this study and in e.g. Anderegg et al. (2010) and Doran and Kendall-Zimmermann (2009) – that the more expert scientists report stronger agreement with the IPCC position. Moreover, on the question of how likely the greenhouse contribution exceeded 50%, many respondents provided a stronger statement than was made in AR4. Using a smaller sample of scientists, Bray (2010) found no difference in level of consensus between IPCC authors and non-authors.

9. How reliable are the responses regarding the respondent’s area of expertise and number of peer-reviewed publications?

Respondents were tagged with expertise fields, though these were in many cases limited and not meant to be exhaustive. These tags were mainly used to ensure that the group of respondents was representative of the group that the survey was sent to. A subset of respondents was also tagged with a Google Scholar metric. Those who were tagged as “unconvinced” reported more expertise fields than the total group of respondents and also a higher number of publications compared to their Google Scholar metrics, if available (see Supplemental Information).

10. Since most scientists agree with the mainstream and therefore most media coverage is mainstream, what is the problem with “false balance”?

Scientists with dissenting opinions report receiving more media attention than those with mainstream opinions. This results in a skewed picture of the spectrum of scientific opinion. Whether that is problematic is in the eye of the beholder, but it may partly explain why public understanding lags behind scientific discourse (e.g. the “consensus gap”).

Survey Respondents

11. How many responses did you get to the survey?

Out of 6550 people contacted, 1868 filled out the survey (either in part or in full).

The vast majority of invitees are scientists who published peer-reviewed articles about some aspect of climate change (this could be climate science, climate impacts, mitigation, etc.). Not all of them necessarily see themselves as climate scientists.

14. Why did you invite non-scientist skeptics to take part in the survey?

They were included in the survey to ensure that the main criticisms of climate science would be included. They constitute approximately 3% of the survey respondents. Viewpoints that run counter to the prevailing consensus are therefore somewhat magnified in our results.

15. How representative are the survey responses of the “scientific opinion”?

It’s difficult to ascertain the extent to which our sample is representative, especially because the target group is heterogeneous and hard to define. We have chosen to survey the wider scientific field that works on climate change issues. Due to the criteria we used and the number of people invited we are confident that our results are indeed representative of this wider scientific field studying various aspects of global warming. We checked that those who responded to the survey were representative of the larger group of invitees by using various pieces of meta-information.

16. Did you take into account varying levels of expertise of respondents?

Respondent were asked to list their area(s) of expertise and their number of peer-reviewed publications. These and other attributes were used to interpret differences in responses.

17. How did you prevent respondents from manipulating the survey results, e.g. by answering multiple times?

An automatically generated, user specific token ensured that respondents could only respond once.

18. How did you ensure respondent anonymity?

Survey responses were analyzed by reference to a random identification number.

The survey questions are related to physical science issues which are a frequent topic of public debate about climate change.

21. Was the survey reviewed before it was sent to respondents?

Yes, before executing the survey it has been extensively tested and commented on by various climate scientists, social scientists and science communicators with varying opinions, to ensure that questions were both clear and unbiased. Respondents were not steered to certain answers.

Please keep discussions on this thread limited to what is mentioned in this FAQ and to other questions you may have about the survey or the article. Discussion of the survey results should be directed at the more generic blog post.

A survey among more than 1800 climate scientists confirms that there is widespread agreement that global warming is predominantly caused by human greenhouse gases.

This consensus strengthens with increased expertise, as defined by the number of self-reported articles in the peer-reviewed literature.

The main attribution statement in IPCC AR4 may lead to an underestimate of the greenhouse gas contribution to warming, because it implicitly includes the lesser known masking effect of cooling aerosols.

Self-reported media exposure is higher for those who are skeptical of a significant human influence on climate.

In 2012, while temporarily based at the Netherlands Environmental Assessment Agency (PBL), my colleagues and I conducted a detailed survey about climate science. More than 1800 international scientists studying various aspects of climate change, including e.g. climate physics, climate impacts and mitigation, responded to the questionnaire. The main results of the survey have now been published in Environmental Science and Technology (doi: 10.1021/es501998e).

Level of consensus regarding attribution

The answers to the survey showed a wide variety of opinions, but it was clear that a large majority of climate scientists agree that anthropogenic greenhouse gases are the dominant cause of global warming. Consistent with other research, we found that the consensus is strongest for scientists with more relevant expertise and for scientists with more peer-reviewed publications. 90% of respondents with more than 10 climate-related peer-reviewed publications (about half of all respondents), agreed that anthropogenic greenhouse gases (GHG) are the dominant driver of recent global warming. This is based on two different questions, of which one was phrased in similar terms as the quintessential attribution statement in IPCC AR4 (stating that more than half of the observed warming since the 1950s is very likely caused by GHG).

Figure 1. The more publications the respondents report to have written, the more important they consider the contribution of greenhouse gases to global warming. Responses are shown as a percentage of the number of respondents (N) in each subgroup, segregated according to self-reported number of peer-reviewed publications.

Literature analyses (e.g. Cook et al., 2013; Oreskes et al., 2004) generally find a stronger consensus than opinion surveys such as ours. This is related to the stronger consensus among highly published – and arguably the most expert – climate scientists. The strength of literature surveys lies in the fact that they sample the prime locus of scientific evidence and thus they provide the most direct measure of the consilience of evidence. On the other hand, opinion surveys such as ours can achieve much more specificity about what exactly is agreed upon and where the disagreement lies. As such, these two methods for quantifying scientific consensus are complementary. Our questions possibly set a higher bar for what’s considered the consensus position than some other studies. Furthermore, contrarian viewpoints were likely overrepresented in our study compared with others.

No matter how you slice it, scientists overwhelmingly agree that recent global warming is to a great extent human caused.

Figure 2. The more publications the respondents report to have written, the more they agree with greenhouse gases being the main contributor to global warming (red bars). IPCC AR4 authors report the most agreement with GHG-driven global warming. Results are based on two different questions, one about the qualitative contribution of GHG (Q3) and one about the quantitative contribution of GHG (Q1).The latter question resulted in more “undetermined” answers (unknown, I don’t know, or other), presumably because it was more difficult to answer. Hence the percentage of consensus can best be compared by excluding these “undetermined” answers. Responses are shown as a percentage of the number of respondents in each subgroup.

By phrasing Question 1 analogously to the well-known attribution statement of AR4 we found something peculiar: Respondents who were more aware of the cooling effect of aerosols in greater numbers assessed the greenhouse gas contribution to recent warming to be larger than the observed warming (consistent with the IPCC assessments). We concluded that the AR4 attribution statement may lead people to underestimate the isolated greenhouse gas contribution. The comparable AR5 statement is an improvement in this respect.

Media exposure

Respondents were also asked about the frequency of being featured in the media regarding their views on climate change. Respondents who thought climate sensitivity was low (less than 1.75 degrees C per doubling of CO2) reported the most frequent media coverage. Likewise, those who thought greenhouse gases had only made an insignificant contribution to observed warming reported the most frequent media coverage. This shows that contrarian opinions are amplified in the media in relation to their prevalence in the scientific community. This is related to what is sometime referred to as “false balance” in media reporting and may partly explain the divergence between public and scientific opinion regarding climate change (the so-called “consensus gap”).

Figure 3. The most frequent media coverage is reported by respondents who estimated climate sensitivity to be lowest (<1.75 degrees for a doubling of CO2 concentration). Respondents are grouped according to their estimate of climate sensitivity (colored bars). Responses are shown as a percentage of the number of people (N) in each climate sensitivity range.

Survey respondents

Respondents were selected based on a few criteria: Having authored articles with the key words ‘global warming’ and/or ‘global climate change’, covering the 1991–2011 period via the Web of Science. This is the same database used by Cook et al in their recent ERL study (PS: John Cook is co-author on this current study as well). Respondents were also selected based on inclusion in the climate scientist database assembled by Jim Prall, as well as by surveying the recent climate science literature. Prall’s database includes signatories of public statements disapproving of mainstream climate science. They were included in our survey to ensure that the main criticisms of climate science would be included. This last group amounts to less than 5% of the total number of respondents, about half of whom only published in the gray literature on climate change.

Survey questions

Detailed questions were posed about a variety of physical climate science issues, which are discussed in the public debate about climate change. Answer options reflected a variety of viewpoints, all of which were phrased as specific and neutral as possible. Before executing the survey, questions and answers (pdf) were reviewed by physical and social scientists and climate change public commentators with a wide range of opinions (see acknowledgements for a list of names), to minimize the chance of bias.

Comments on the survey by respondents varied: some said it was slanted towards the ‘alarmist’ side (“Obviously these questions were posed by warmists”), but more respondents commented that they thought it was slanted towards the ‘skeptical’ side (“I suspect this survey comes from the denial lobby”).

]]>https://ourchangingclimate.wordpress.com/2014/08/11/survey-confirms-scientific-consensus-on-human-caused-global-warming/feed/38BartVerheggen et al - Figure 1 - GHG contribution to global warmingConsensus results - from Fig 3 Verheggen et alVerheggen et al - Figure S13c - media exposure vs ECS estimateOpen thread Summer 2014https://ourchangingclimate.wordpress.com/2014/08/11/open-thread-summer-2014/
https://ourchangingclimate.wordpress.com/2014/08/11/open-thread-summer-2014/#commentsMon, 11 Aug 2014 11:37:08 +0000http://ourchangingclimate.wordpress.com/?p=2722]]>For all climate-related discussions that don’t fit under a recent thread.]]>https://ourchangingclimate.wordpress.com/2014/08/11/open-thread-summer-2014/feed/32BartClimateDialogue on Climate Sensitivityhttps://ourchangingclimate.wordpress.com/2014/05/15/climatedialogue-on-climate-sensitivity/
https://ourchangingclimate.wordpress.com/2014/05/15/climatedialogue-on-climate-sensitivity/#commentsThu, 15 May 2014 19:38:23 +0000http://ourchangingclimate.wordpress.com/?p=2695]]>After a bit of a “hiatus”, ClimateDialogue (CD) has re-opened again with a discussion on climate sensitivity. On the one hand this site is unique in bringing together ‘mainstreamers’ and ‘contrarians’ (both in the organization and in the discussions), hopefully leading to both enhanced clarity on what the (dis)agreements are really about and to decreased polarization. On the other hand it’s controversial because a ‘false balance’ is embedded in its structure (by purposefully inviting contrarian scientists to the discussion, rather than e.g. randomly inviting experts).

Whether the positives or negatives dominate is in the eye of the beholder (opinions about that vary wildly), but also depends very strongly on the participation of the mainstream (both as invited experts and as contributing to the public discussion). See also my initial reflections at the time of the first launch. Discussions on ClimateDialogue will be facilitated and moderated by Bart Strengers (NL Environmental Assessment Agency, PBL) and Marcel Crok (freelance journalist), where the former has a mainstream view of climate science and the latter a contrarian view. I am still involved in the background, as is KNMI (NL Meteorological Institute). ClimateDialogue is funded by the Dutch Ministry of Infrastructure and Environment.

In the current ‘dialogue’ James Annan, John Fasullo and Nic Lewis are discussing their views about climate sensitivity (the equilibrium warming after a doubling of CO2 concentrations, ECS). In the latest IPCC report (AR5) the different and partly independent lines of evidence are combined to conclude that ECS is likely in the range 1.5°C to 4.5°C with high confidence. The figure below shows the ranges and best estimates of ECS in AR5 based on different types of studies, namely:

The pros and cons of these different lines of evidence are discussed, as well as the weighing of the evidence and the resulting range and best estimate.

Fasullo argues for “combined consideration of the individual approaches” and for an ECS range between 2 and 4.5 degrees C (based on recent evidence since the publication of AR5 pointing to the middle of this range).

Annan writes: “The recent transient warming (combined with ocean heat uptake and our knowledge of climate forcings) points towards a “moderate” value for the equilibrium sensitivity, and this is consistent with what we know from other analyses. Overall, I would find it hard to put a best estimate outside the range of 2-3°C.”

Lewis prefers a subset of studies in the “instrumental” category over model-based and paleo-climate approaches and conclude that “the soundest observational evidence seems to point to a best estimate for ECS of about 1.7°C, with a ‘likely’ (17-83%) range of circa 1.2–3.0°C.”

Nic Lewis appears to be arguing primarily on the basis that all work on climate sensitivity is wrong, except his own, and one other team who gets similar results. In reality, all research has limitations, uncertainties and assumptions built in.

A few recent studies (Shindell, 2014; Kummer and Dessler, 2014) suggest that the “efficacy” of cooling aerosols may be larger than expected, because they are largely confined to the landmasses of the Northern hemisphere. This could explain the difference in ECS as deduced from the instrumental period vs from climate models and paleo-climate. After all, such differences need explaining. If correct, this would mean that ECS is indeed somewhere in the middle of the IPCC range (~3 degrees C). In turn, this would mean that the warming should pick up speed again in the near future, James Annan argues, since the current rate of warming is close to the lower end of the model range:

If these results are correct, then the current moderate warming rate is a bit of an aberration, and so a substantial acceleration in the warming rate can be expected to occur in the near future, sufficient not only to match the modelled warming rate, but even to catch up the recent lost ground.

However, Schmidt et al (2014) show that when accounting for the timing of ENSO events, for the actual evolution of climate forcings (e.g. volcanic aerosols) and for inaccuracies in global temperature datasets, observations and the model ensemble mean are very similar. That weakens Annan’s conclusion about the future warming trend, as cited just above.

Like at CCNF, there is a public discussion alongside the ‘invitee-only’ discussion, so feel free to chime in (registration required). You may also contact one of the editors (Bart S, Marcel C) or me to bring something into the invitee’s discussion.

There are similarities between ClimateDialogue and ClimateChangeNationalForum (CCNF, with which I’m also affiliated) e.g. in trying to make scientific discussions visible to the public, though there are also important differences. E.g. CCNF has a more open discussion structure, whereas CD has moderated discussions between a smaller set of invited participants. At CCNF the set of science columnists is more reflective of the scientific spectrum of opinion than at CD (though CCNF has also been criticized for “amplifying nonsense”; see also the recent discussion at CCNF itself). This post has also been published on CCNF.

As a tongue-in-cheek epilogue, let me end with this hilarious clip from “Last week tonight” with John Oliver, in which he contrasts the common ‘false balance’ with what a ‘statistically representative climate debate’ would look like:

As Jeff Nesbit tweeted: “Being the last scientist to accept established climate science doesn’t make you Galileo.” Quite the opposite indeed.

The Galileo-complex also suggests a rather simplistic view of how science progresses. Rather than a lone skeptic overthrowing a scientific (rather than a cultural) consensus, scientific progress is a usually a gradual process. New evidence has to be reconciled with the existing mountain of evidence; it doesn’t simply replace it. Observing a bird in the air doesn’t disprove gravity. “Skeptics” and their supporters often bring up Galileo as an example of that the scientific consensus can also be wrong, and has been wrong in the past. True enough, though as Carl Sagan said: “they laughed at Galileo, but they also laughed at Bozo the clown”.

“Yes, the hot spot is expected via the traditional view that the lapse rate feedback operates on both short and long time scales. (…) it [the hot spot] is broader than just the enhanced greenhouse effect because any thermal forcing should elicit a response such as the “expected” hot spot.”

So why is he claiming something in the WSJ that he knows to be untrue?

But rather than doing a careful analysis of various potential explanations, McNider and Christy, as well as their colleague Roy Spencer, prefer to draw far reaching conclusions based on a particularly flawed comparison: They shift the modelled temperature anomaly upwards to increase the discrepancy with observations by around 50%. Using this tactic, Roy Spencer showed the following figure on his blog recently:

So what did he do? Jos Hagelaars tried to reproduce the different steps involved. A comparison of annual data, using a 1986-2005 baseline, would look as follows:

Spencer used a 5 year running mean instead of annual values, which would (should) look as follows:

The next step is re-baselining the figure to maximize the visual appearance of a discrepancy: Let’s baseline everything to the 1979-1983 average (way too short of a period and chosen very tactically it seems):

Victor writes about the meme regularly used by the anti climate science campaign, often supported by some straw man arguments, that the science of human impacts on climate would not be falsifiable. He shows it’s nonsense, by giving some examples of how it could be falsified. Or, more likely, already would have been falsified, if the science would be wrong. Victor’s post inspired me to think of more options to falsify generally accepted viewpoints in climate science. If there are any ‘climate change skeptics’ who want to contribute to real science, they might see this as a challenge. Maybe they can come up with a research proposal, based on one of the options for falsification. Like proper scientists would do.

First, a few more things about falsifiability in general. Bart wrote a concise post about the subject four years ago, explaining that a bird in the sky does not disprove gravity. What looks like a refutation at first, might on second thoughts be based on partial or total misunderstanding of the hypothesis. Natural climate forcings and variations do not exclude human impacts. Therefore, the existence of these natural factors in itself, cannot falsify anthropogenic climate change. A real skeptic is cautious about both scientific evidence and refutations. ‘Climate change skeptics’ like to mention the single black swan, that disproves the hypothesis that all swans are white. Of course that is true, unless that single black swan appears to be found near some oil spill.

Some of the falsifications that I mention later on might be somewhat cheap, or far-fetched. It is not very easy to find options to falsify the science of human impacts on climate. Not because climate scientists don’t respect philosophical principles of science, but simply because there’s such a huge amount of evidence. There are not a lot of findings that would disprove all the evidence at once. A scientific revolution of this magnitude only happens very rarely. Whoever thinks differently, doesn’t understand how science works.

Karl Popper

Even more, the claim ‘The AGW hypothesis is unfalsifiable’ demonstrates a lack of understanding of Popper’s ideas, in which falsifiability is so important. I don’t think Popper’s philosophy implies that some three word hypothesis – Anthropogenic Global Warming – can be rejected by nothing but a few simple claims. Popper would expect a more serious intellectual effort from a scientist. First, he will have to find an accurate wording for his hypothesis. The next step is some thorough thinking about the consequences. This will help him to design tests that can either support or falsify his idea. If, in the end, the result of the test appears to be worthwhile, the scientist will write a paper on this whole enterprise.

As a matter of fact, the ‘AGW-hypothesis’ is not a hypothesis in the Popperian sense. The human impact on climate is a theory, supported by many hypotheses, each of them tested according to widely accepted scientific standards. Just as Popper and his successors in the philosophy of science would have wanted.

One more thing. The philosophical principle of falsifiability and the feasibility of tests for it are two different subjects. Scientists are still busy testing some of the implications of Einsteins ideas, because the technology did not exist in Einstein’s days. And it is highly unlikely that the scientists that proposed the Higgs boson ever even dreamed of the Large Hadron Collider, because it was beyond anyone’s imagination at the time. Philosophy of science does not set a time frame for hypotheses testing. The issues involved in the testing of hypotheses are the story of almost every scientist’s life. They’re not sitting back, thinking of new and brilliant ideas, most of the time. Instead, they are busy digging for data, messing with measuring equipment, or evaluating errors in experiments. For climate scientists, one of the major issues is the pace at which they can get new information: one year of data every year. And one year of data is not a lot for climate research. There are no test tube planets for climate experiments. They will have to do with what is left: observations of (changes in) the climate in the present and the past and simulations of the relevant processes in the climate system in computer models. Most self-proclaimed skeptics seem to have objections to the latter as well. Wouldn’t it be nice if they, just for a change, would say how it should be done?

That’s it for falsifiability in general. Here are the 10 way to disprove the human impact on climate.

1. A drop in global temperatures for some period of time to the level of 50 years ago or longer, without a clear cause

The average global temperature is almost 1 °C higher now than it was in the early 20th century. The widget by Skeptical Science (which unfortunately does not work very well in a WordPress blog) adds some perspective to the amount of energy accumulating in the climate system. These huge amounts of energy do not simply stay in the climate system without a cause. It is what we expect to happen, based on the greenhouse theory. And there’s no other explanation that is supported by a reasonable amount of evidence. It would be very clear that science overlooked something important, if all the energy would suddenly escape without something extraordinary happening (like a huge volcanic eruption), or hide in some unknown place.

2. A drop in global sea level for some period of time

There are two major causes for sea level rise: thermal expansion of seawater and the melting of land ice. Water extraction from and (temporal) accumulation on land play a minor role. At this moment, thermal expansion is the main factor. This is evidence for warming of the oceans, which is important because the oceans can store much more heat than the atmosphere. Thus, the ocean level falling, would not only be evidence for cooling of the oceans; it would be strong evidence that the climate system as a whole would be losing energy. (Note: pure water has a strange property: the density decreases with an increase in temperature between 0 and 4 °C. This effect disappears with increasing salinity. For almost all sea water, the maximum density is at freezing point).

Changes in sea level on a short term are not caused by thermal expansion or contraction, so they do not falsify anthropogenic climate change. The figure below, from the University of Colorado’s Sea Level Research Group, for instance, shows a substantial seasonal variation. This variation can be filtered out of that data, and that’s the graph that is usually shown.

3. A strong rise or decline in the atmospheric CO2 level

Since the late 50’s, the Keeling Curve shows an ongoing rise in the amount of CO2 in the atmosphere. If there would be a sudden huge change in the CO2 level, without a clear, demonstrable cause, that would be proof that our knowledge of the carbon cycle falls short.

Climate change skeptics sometimes refer to a graph by Ernst-Georg Beck, in which thousands of megatons of CO2 mysteriously seem to appear in atmosphere within a few years, and then disappear again, causing wild fluctuations in CO2 levels. The fluctuations miraculously stop in 1958, exactly when Keeling started his measurements on Mauna Loa. Maybe we are being fooled for more than half a century by all CO2 molecules in the world. But it’s more likely that the graph below, from Cripps, father and son Keeling’s home base, displays the more accurate data.

4. The discovery that climate forcings in the past were much larger, or temperature changes much smaller, than science thinks

One of the ways to estimate climate sensitivity, is by looking at temperature changes in the past and the knowledge of their causes. It is very likely that the magnitude of a temperature change mostly depends on the magnitude of a change in the radiation balance at the top of the atmosphere, rather than the exact cause of this change. In other words: a Watt per square meter is a Watt per square meter, no matter if it comes from the sun, from an increased greenhouse effect, or something else. So, smaller temperature changes in the past, of larger forcings causing them, would be evidence for a low climate sensitivity.

Climate change skeptics often claim that relatively small change in the radiations balance are responsible for a significant part of the warming that we’ve seen since the early 20th century, or for temperature changes in a more distant past. They don’t seem to realize that these claims imply a higher, rather than a lower climate sensitivity than is generally assumed by scientists.

5. Warming of the stratosphere

Many changes that are happening in the climate system are caused by warming itself. Observations of these changes cannot be used as evidence for the cause of warming. But there are some changes – fingerprints – that are specific for the increased greenhouse effect. Cooling of the stratosphere is one of these fingerprints. This cooling is confirmed by measurements, as is shown in the figure below, from ‘State of the Climate 2012‘ by the Bulletin of the American Meteorological Society.

We can see the absorption of heat by greenhouse gases in satellite measurements of longwave radiation that leaves the earth’s atmosphere. The absorption bands of CO2, methane, ozone and water vapor are clearly visible in these measurements, as shown in the figure below. Whoever can demonstrate the measurements to be wrong, will make it into the history books. It would not only disprove the human impact on climate, it would wipe quite a lot of established physical science off the table.

7. Evidence of a substantial fall of relative humidity with rising temperature

If specific humidity would not follow temperature, the relative humidity would be lower in a warmer world. Then, there would not be a positive water vapor feedback, or it would be very small. It is highly likely that this would make matters rather worse than better. Our greenhouse gas emissions would lead to a smaller rise in temperatures than expected, but the downside would probably be disastrous: world-wide drought. Unless Clausius and Clapeyron were wrong.

8. A source of heat in the climate system that we do not know yet

All the evidence shows the heat in the climate system has been increasing for decades, and still is. Assuming even climate change skeptics do not dispute the law of conservation of energy, there has to be a source of heat somewhere. Who knows, one day, we might find some kind of mini-sun, hidden deep in the oceans. It would be a game changer for climate science.

9. A fundamental flaw in the scientific understanding of radiation physics or thermodynamics

This one is especially for the ‘slayers’, who deny that there is a greenhouse effect at all. Their ideas are either utter nonsense, or they are about to discover the very biggest mistake in the history of science. It would mean that we’d have to reevaluate fundamental physical science, that has been undisputed for decades to centuries, like the Stefan-Boltzman law or even the laws of thermodynamics. We would probably end up rewriting every single physics book in the world.

10. CO2 molecules appear to behave differently in the wild, than they do in a laboratory

I added this last one as a ‘tribute’ to one of the veterans of the war on climate science in The Netherlands. He, whose name I will not mention, does not dispute the greenhouse effect, but thinks it is relevant to mention that absorption of longwave radiation by CO2 has only been measured ‘in laboratory conditions’. He’s wrong, of course, because there are satellite measurements as well. But let’s forget about that. I think the idea of molecules behaving differently in the lab, compared to their behavior in the wild, is so creative that it deserves attention. I won’t go into the consequences of this revolutionary hypothesis. I will leave that, dear reader, to your imagination.

]]>https://ourchangingclimate.wordpress.com/2014/02/17/is-climate-science-falsifiable/feed/591joshagelaarspopper3sl_globalco2_800k_zoomStratostemplw_absorptionsat_vapor_pressureThe role of scientific consensus in moving the public debate forwardhttps://ourchangingclimate.wordpress.com/2014/02/07/scientific-consensus-public-debate-hulme-lewandowsky-cook/
https://ourchangingclimate.wordpress.com/2014/02/07/scientific-consensus-public-debate-hulme-lewandowsky-cook/#commentsFri, 07 Feb 2014 20:54:26 +0000http://ourchangingclimate.wordpress.com/?p=2621]]>Mike Hulme had an interesting essay at The Conversation, the main message of which was

In the end, the only question that matters [for the public debate about climate change] is, what are we going to do about it?

Hulme correctly argues that the basic science is clear enough so that for society the important issues to discuss are not science related, but policy related. I argued much the same here. He writes:

Let’s leave the minor quibble aside that AR5 puts the anthropogenic contribution at ‘extremely likely’ having caused more than half of the recent global warming.

The part where I disagree with Hulme is where he argues that showing the existence of a scientific consensus on the above (it is warming; it’s due to us; it’s bad news) somehow stands in the way of getting society to discuss that most important question. I think the opposite is true. It is the continuous doubt about the science, sowed by those who oppose a serious discussion about what to do, that is a stumbleblock. Showing that a consensus amongst experts exists would enable society to more swiftly move on to the important conversation on what to do about it. I agree with Hulme that on this deeply ethical question there is, and ought to be, a multitude of opinions.